Traveling wave tubes



July 24, 1962 E. c. DENCH TRAVELING WAVE TUBES 2 Sheets-Sheet 1 Filed Sept. 30, 1958 July 24, 1962 E. c. DENCH TRAVELING WAVE TUBES 2 Sheets-Sheet 2 Filed Sept. 30, 1958 M/VEN 707? ED WA RD 6. DEA/Ch ATTORNEY United States This invention relates to a traveling wave electron dis charge device in which certain parameters of the slow wave propagating structure thereof are progressively varied so as to provide an impedance match between the slow wave structure and an external coupling means,

Traveling wave tubes which depend upon interaction between an electron beam and high frequency fields of wave energy propagating along a slow wave structure or delay line are well known. In these tubes, energy may be transferred from the electron beam to the high frequency field when the electron beam velocity is substantially synchronized with the phase velocity of the high frequency field. A signal introduced into one end of such a delay line may be amplified, or the tube under certain conditions may be made to oscillate by virtue of this energy exchange.

One of the problems involved in traveling wave tubes designed for operation at relatively low frequencies is that of matching the impedance of the slow wave structure or delay line with the impedance of an input or output coupling means. For example, the impedance of an interdigital delay line may be of the order of 150 ohms, while that of a coaxial input or output coupling means is of the order of 50 ohms. If the delay line configuration atQIlt ice above obviously would be reversed.

along its entire length is unchanged, it is necessary, in

order to obtain the desired impedance match, to use a transition means, such as a quarter wave transformer, positioned external to the delay line between the end of the delay line and the coaxial coupling means. Since the length of a quarter wave transformer is inversely proportional to frequency, such a transition means becomes unduly large at frequencies in the range of about 500- to 1500 megacycles, or lower. For example, at 500* megacycles, a quarter wave length is 15 centimeters; consequently, the length of the external coupling means, exclusive of the coaxial line, would be approximately six inches. The length of the coaxial conductor required for coupling energy into or from the delay line at such frequencies becomes excessive, particularly in applications requiring a minimum of space for equipment.

In accordance with this invention, the impedance of the traveling wave tube delay line is matched to that of the external coupling means by progressively altering certain parameters of the interdigital delay line. The impedance of an interdigital delay line increases as the space between adjacent edges of adjacent fingers increases and decreases with decreased separation between fingers. This finger separation may be accomplished in two ways. Firstly, the pitch, that is the distance between corresponding points on two adjacent fingers, may be held constant while the finger thickness is altered. Secondly, the finger thickness may be maintained constant while the pitch is varied. Both methods of varying the finger separation, of course, may be employed in the same traveling wave tube.

The impedance of the interdigital delay line also increases as the height of the fingers decreases, and vice versa. :The finger height is the dimension of the finger measured from the tip to the edge adjacent the back wall of the delay line. This last method of varying impedance may be used together with any one of the two alternative methods previously mentioned.

If the delay line impedance is greater than that of the external coupling means, the transition region of delay line between the major portion of the delay line and the external coupling means would be characterized by grad- If the traveling wave tube is an oscillator, only one coupling means, namely, the output coupling means and one transition region, would be required; if, on the other hand, the traveling wave tube is an amplifier, an input and output coupling means, as well as a transition region, at or near both ends of the delay structure, would be employed.

The type of coupling means is immaterial to the invention; for example, a coaxial line or a waveguide may be used to couple energy into or out of the traveling wave tube.

Other objects and features of this invention will be understood more fully from the following detailed description of theinvention with reference to the accompanying drawings wherein:

FIG. 1 is a longitudinal cross-sectional view, partly in elevation, of a traveling Wave oscillator which may incorporate a delay line according to the invention;

FIG. 2 is a sectional View, partly in elevation, taken along line 2-2 of FIG. 1;

FIG. 3 is a detail view of a portion of the delay line of the traveling wave oscillator of FIGS. 1 and 2;

FIG. 4 is a fragmentary sectional view of a traveling wave amplifier using the type of delay line construction indicated in FIGS. 2 and 3;

FIG. 5 is a sectional view, partly in elevation, of 'a traveling wave tube showing another type of delay line construction in accordance with the invention;

FIG. 6 is a sectional view, partly in elevation, of a traveling wave tube illustrating one combination of the types of delay line construction shown in FIGS. 2 and 5; FIG. 7 is a fragmentary sectional view of a traveling wave tube illustrating another combination of the types of delay line construction shown in FIGS. 2 and 5; and

FIG. 8 is a sectional view of a traveling wave tube illustrating still another type of delay line construction in accordance with the invention.

Referring now to FIGS. 1 to 3, a traveling wave tube 10,

herein shown as an oscillator, comprises a periodic slow wave energy propagating structure or delay line 12, an elongated electrode 14, referred to as a sole, a lead-in assembly 16, an output coupling'means 18, an electron gun assembly 20, and a magnetic field-producing means 21, a portion of which is illustrated in FIG. 1. The cir- 1 cular delay line 12 includes several interdigit-al fingers or elements 22 which extend from oppositely disposed annular members 24. The latter may be secured, as by screws 26, to the shoulder portion of a cylindrical electrically-conductive ring 207. The remainder of the slow wave structure 12 includes a pair of oppositely disposed cover plates 28 and 29 hermetically sealed to ring 27.

The sole 14, which is arranged concentric with the delay line 12, and which is normally maintained negative with respect to the delay line, consists essentially of a cylindrical block of electrically-conductive material which includes a web portion 31 bounded by an outer section 32 whose periphery consists of an active surface 33 and side members 34. The purpose of the side members 34 is to confine the electron beam within the interaction space 35 between the active surface 33 of sole 14 and the interdigital delay line 12. A tubular metallic insert 37 is brazed into position against the inner periphery of a centrally disposed aperture in the web portion 31 of sole 14. One end of a hollow supporting member 38 is located 3; within the insert 37 and is fixedly attached thereto. Supporting member 38, in addition to providing support for the sole 14, forms a portion of lead-in assembly 16 and allows for passage of external circuit connecting leads in a manner to be described subsequently.

Sole 14 contains a slot 27 for accommodating the electron gun assembly 20. Since the invention does not involve details of the electron gun, the latter is shown schematically in the drawing. The construction and manner of mounting of the electron gun may be as shown in a copending application of Roy A. Paananen, Serial No. 717,897, filed February 27, 1958, now Patent No. 2,914,- 700. The electron gun assembly 20 includes a cathode 41, a heater 42, a grid 43 which may be used for control of beam current (as for amplitude modulation, in the case of an oscillator) and an accelerating electrode 44. The cathode 41 may be in the form of a rectangular prism provided with a circular bore in which a folded heater wire 42 is inserted; the heater may be connected at one end to the inner wall of the cathode. Electrical energy from appropriate sources is supplied to the cathode 41, heater 42, grid 43, and accelerating electrode 44 by way of respective lead-in wires 51, 52, 53, and 54, which are brought out from the tube envelope through the lead-in assembly 16.

Lead-in assembly 16 includes an electrically-conductive sleeve 56 attached to the inner periphery of cover plate 29, as indicated in FIG. 1. A section of cylindrical glass tubing 57 interconnects sleeve 56 and a second electricallyconductive sleeve 58. The other end of sleeve 58 is provided with a glass seal 59 for sealing the traveling wave tube 10 after evacuation. One end of the sole-supporting member 38 contains an outwardly flared portion 38' which is connected to the inner surface of sleeve 58. The leads 51, 52, 53, and 54 are mounted in electrically insulated relation with supporting member 38 and form one another by one or more glass beads 55.

The coaxial output coupling means 18 is sealed in an opening of ring 27 of delay line 12 and is impedancematched to the delay line, in a manner to be described later. The inner conductor 58 of the output coupling means 18 is connected to a finger of delay line 12 at or near the end of the delay line adjacent electron gun 20.

Traveling wave tube 10 may be provided with a collector electrode 60 for intercepting electrons after one traversal of the circular interaction space 35. This collector electrode may be in the form of a projection from the back wall 47 of delay line 12 or may be a dependent structure maintained at an electrical potential of about the same order of magnitude as that of the delay line. In some instances, however, the collector electrode may be omitted and the electron stream made reentrant. This type of construction is shown in FIGS. 5, 6, and 8.

The necessary electric field between the slow wave structure 12 and sole 14 may be obtained by means of a unidirectional voltage applied therebetween; such a voltage may be supplied by batteries 71 and 72. The sole 14 may be biased negatively relative to the cathode 41 by means of the source 72 connected between cathode lead 51 and sole supporting member 38 by way of sleeve 58. The cathode 41 may, however, in some instances be at the same potential as the sole, in which case the source 72 would be omitted. Similarly, the delay line 12 may be maintained at a potential positive relative to both sole 14 and cathode 41 by means of the source 71 of unidirectional voltage connected between the cathode and sleeve 56, which sleeve is connected, in turn, to delay line 12. The heater voltage is obtained by means of a source 73 of voltage connected to leads 51 and 52. The accelerating electrode 44 may be maintained at a potential positive relative to the cathode 41 by means of a source 74 of unidirectional voltage connected between leads 51 and 54. The control grid lead 53 may be connected by way of terminal 75 to an appropriate energy source for controlling the magnitude of the electron beam current in the traveling wave tube.

A uniform magnetic field transverse to the direction of propagation of an electron beam is provided by a permanent magnet or electromagnet having cylindrical pole pieces 78 and 79 radially positioned on or adjacent the anode cover plates 28 and 29, respectively. Pole piece 78 is apertured to receive lead-in assembly 16, and pole piece 79 is apertured to maintain symmetry of the magnetic field. The magnetic flux should be concentrated in the interaction space 35 between the sole 14 and the delay line 12 and are generally transverse to the electric field lines between the sole 14 and delay line 12. By proper adjustment of the magnitude and polarity of the magnet and electric fields thus established, the electron beam may be caused to move along a more or less circular path in the interaction space 35 under the combined influence of these transversely disposed fields.

As shown clearly in FIGS. 2 and 3, a major portion of the interdigital delay line 12 according to the invention is of uniform configuration wherein the space between adjacent fingers 22, as well as the height and thickness of the fingers, is uniform. The fingers in this major portion of uniform configuration are indicated simply by the reference numeral 22, in contrast with the fingers in the transition region 23, now to be described, which are referred to by the reference numeral 22 with a letter. The transition region 23 of delay line 12 of FIGS. 2 and 3 comprises fingers 22A 22E wherein the separation between fingers, that is, the dimension n in FIG. 3, or the pitch p between corresponding points on adjacent fingers, is gradually decreased as the output coupling means 18 is approached, while the thickness t of the fingers is maintained constant. In this manner, the impedance of the delay line in the transition region progressively decreases as the distance from the output coupling means decreases.

The same type of delay line construction is shown in FIG. 4, except that the device of FIG. 4, being an amplifier, has an output coupling means 18 at one end of the delay line 12 and an input coupling means 19 at the other end of the delay line. For this reason, both ends of the delay line have transition regions including fingers 22A 22E.

It will be noted that the tube of FIG. 4, in addition to differing from the tube of FIG. 2 in that it is an amplifier rather than an oscillator, further dilfers in that the negative electrode 14 of the tube of FIG. 4 may serve as a continuous cathode and the localized electron gun 20 of FIG. 2 is omitted. The continuous cathode of the tube of FIG. 4 may be used with the oscillator of FIG. 2 or the localized electron gun 29 of 'FIG. 2 may be used with the amplifier of FIG. 3. In other words, the type of electron beam source used is not a part of the invention and does not depend upon the type of delay line transition region employed. It should also be noted that the collector electrode 60 of FIG. 2 may be used with a continuous cathode of the type shown in FIG. 4 when a non-reentrant electron beam is desired.

Although there are six fingers shown in the transition region of the device of FIGS. 2 and 3, and five fingers illustrated in the transition region of the device shown in FIG. 4, the number of such fingers is not limited to any particular number but depends upon the various parameters of the tube and of the external coupling means, such as the relative impedance of the uniform portion of the delay line 12 and the coupling means, the size of the fingers of the delay line, which may be dictated, to some extent, by the power capability of the tube, etc. Moreover, the taper of the interdigital delay line 12 in the devices of FIGS. 2 to 4, as well as those of FIGS. 5 to 7, may be linear, exponential, or any other configuration which permits the characteristic impedance of the delay line to change gradually along the transition region as the external coupling means is approached.

FIG. 5 illustrates another type of transition region 23 whereby the characteristic impedance of the delay line r may be progressively varied. Here, as in the delay lines of FIGS. 2 to 4, the fingers 22A 22E in the transition regions are constructed so that the finger separa tion, and consequently, the impedance of the transition region of the delay line 12, progressively decreases as the external coupling means is approached. In contrast with the delay lines of FIGS. 2 and 4, the decrease in finger separation in FIG. 5 is obtained by maintaining the pitch constant while varying the finger thickness. If the impedance of the output coupling means Were greater than that of the delay line, the finger thickness would be made to decrease in the direction of the output coupling means; in other words, the impedance of the transition region would increase in the direction of the output coupling means. The type of transition described in FIG. 5, may, like that previously referred to, be incorporated in an amplifier as well as in an oscillator; in such a case, a transition region would be located at both ends of the delay line 12, in the manner shown in FIG. 4.

In FIG. 6, the transition region of the delay line 12 incorporates one possible combination of the two types of progressive variation in finger separation previously described. The portion of the transition region including of construction, materials and processes described, as

' many equivalents will suggest themselves to those skilled fingers 22A, 22B, and 220 is characterized by variable pitch and constant finger thickness, as in the device of fingers 2 and 3, while the portion of the transition region including fingers 22D 22H is characterized by constant pitch and variable finger thickness, as in the device shown in finger 5. The configuration of the transition region of the delay line in FIG. 6 illustrates only one of many possible combinations of the two methods of achieving variable finger separation already described. For example, the fingers 22A.. 22C may be of constant pitch and variable finger thickness and the fingers 22D 22H may be of constant finger thickness and variable pitch. Furthermore, there could be more than one group of fingers of each type of construction in the transition region of the delay line. The arrangement shown in FIG. 6 is equally applicable to a traveling wave amplifier.

Another possible combination of the two types of progressive variation in the transition region previously described is illustrated in FIG. 7. The portion of the transition region including fingers 22A 22B is characterized by a simultaneous variation of finger thickness and pitch, the pitch being measured with reference to the center lines of the fingers. As shown in FIG. 7, the spacing between center lines of the fingers 22A 22E progressively increases as the external coupling means 18 is approached and, concurrently, the thickness of the fingers 22A 22E progressively increases as the output coupling means 18 is approached. The center line-to-center line spacing and thickness may be progressively decreased as the output coupling means 18 is approached in cases where the characteristic impedance of the delay line 12 is less than that of the coupling means. The arrangement of the transition region shown in FIG. 7 is applicable to amplifiers, as well as to oscillators.

In FIG. 8, a traveling wave tube is shown wherein the delay line transition region between the uniform portion of the delay line and the coaxial output coupling means includes fingers 22A 22F whose height h is in creased progressively as the output coupling means 18 is approached. In this manner, the impedance of the transition region of the delay line 12 decreases as the output coupling means is approached. If the impedance of the delay line should be less than that of the output coupling means, however, the height of the fingers 22A 22F would decrease progressively rather than increase in the direction of the-output coupling means.

The configuration of the transition region 'o-f the delay line Y 12 of FIG. 8 may be used at both ends of the delay line in the event that the tube of FIG. 8 is a traveling Wave amplifier.

This invention is not limited to the particular details in the art. It is accordingly desired that the appended claims be given a broad interpretation commensurate with the scope of the invention within the art.

What is claimed is: .7

I. In combination, a periodic interdigital slow wave energy propagating structure having a plurality of interdigital fingers and constructed to produce along a path adjacent thereto fields of electromagnetic wave energy being transmitted, means for directing a beam of electrons along said path in energy-exchanging relation with said wave fields, and external energy coupling means coupled to said structure, said structure including a major region and a transition region disposed between said major region and said external energy coupling means, the dimensions of the fingers transverse to said path in said transition region being progressively varied, said energy coupling means being coupled directly to one of said fingers.

2. A traveling wave oscillator comprising a periodic interdigital slow wave energy propagating structure having a plurality of interdigital fingers and constructed to produce along a path adjacent thereto fields of electromagnetic wave energy being transmitted, means for directing a beam of electrons along said path in energy exchanging relation with said wave fields, and external energy output coupling means coupled to said structure adjacent the end thereof away from which electrons move along said path, said structure including a major region and a transition region disposed between said major region and said external energy coupling means, the dimensions of the fingers transverse to said path in said "transition region beingprogressively varied, said energy coupling means being coupled directly to one of said fingers.

3. In combination, aperiodic interdigital slow wave energy propagating structure having a plurality of interdigital fingers'and constructed to produce along a path adjacent thereto fields of electromagnetic wave energy being transmitted, means for directing a beam of electrons along said path in energy-exchanging relation with said wave fields, an external energy coupling means coupled to said structure, said structure including a major region and a transition region disposed between said major region and said external energy coupling means, the pitch of said transition region being constant and the height of the fingers in said transition region being progressively varied, said energy coupling means being coupled directly to one of said fingers.

'4. In combination, a periodic interdigital slow wave energy propagating structure having a plurality of interdigital fingers and constructed to produce along a path adjacent thereto fields of electromagnetic wave energy being transmitted, means for directing a beam of electrons along said path in energy-exchanging relation with said wave fields, and external energy coupling means coupled to said structure, said structure including a major region and a transition region disposed between said major region and said external energy coupling means, the thickness of the fingers in said transition region being maintained constant and the pitch and height of the fingers in said transition region being progressively varied, said energy coupling means being coupled directly to one of said fingers.

5. In combination, a periodic interdigital slow wave energy propagating structure having a plurality of interdigital fingers and constructed to produce along a path adjacent thereto fields of electromagnetic wave energy being transmitted, means for directing a beam of electrons along said path in energy-exchanging relation with said wave fields, and external energy coupling means coupled to said structure, said structure including a major region and a transistion region disposed between said major region and said external energy coupling means, the height and the distance between adjacent edges of adjacent fingers in said transition region being progressively varied, said energy coupling means being coupled directly to one of said fingers.

6. in combination, a periodic interdigital slow wave energy propagating structure having a plurality of interdigital fingers and constructed to produce along a path adjacent thereto fields of electromagnetic wave energy being transmitted, means for directing a beam of electrons along said path in energy-exchanging relation with said wave fields, and external energy coupling means coupled to said structure, said structure including a major region and a transition region disposed between said major region and said external energy coupling means, the distance between center lines of said fingers in said transition region and the height of said fingers in said transition region being simultaneously and progressively varied, said energy coupling means being coupled directly to one of said fingers.

7. In combination, a periodic interdigital slow wave energy propagating structure having a plurality of interdigital fingers and constructed to produce along a path adjacent thereto fields of electromagnetic wave energy being transmitted, means for directing a beam of electrons along said path in energy-exchanging relation with said wave fields, and external energy coupling means coupled to said structure, said structure including a major region and a transition region disposed between said major region and said external energy coupling means, said transition region including at least one portion wherein the thickness of said fingers is maintained constant and the pitch of said fingers is progessively varied, said transition region including at least one other portion wherein the pitch and height of said fingers is held constant and the height of said fingers is progressively varied, said energy coupling means bieng coupled directly to one of said fingers.

8. In combination, a periodic interdigital slow wave energy propagating structure having a plurality of interdigital fingers and constructed to produce along a path adjacent thereto fields of electromagnetic wave energy being transmitted, means for directing a beam of electrons along said path in energy-exchanging relation with said wave fields, and external energy coupling means coupled to said structure, said structure including a major region and a transition region disposed between said major region and said external energy coupling means, the height of the fingers in said transition region being progressively varied.

9. In combination, a periodic interdigital slow wave energy propagating structure having a plurality of interdigital fingers and constructed to produce along a path adjacent thereto fields of electromagnetic Wave energy being transmitted, means for directing a beam of electrons along said path in energy-exchanging relation with said wave fields, external energy coupling means coupled to said structure, said structure including a major region, and means for matching the characteristic impedance of said major region to the characteristic impedance of said energy coupling means, said means for matching including a transition region disposed between said major region and said energy coupling means in which the dimensions of said fingers transverse to said paths are progressively varied so that the characteristic impedance of said transition region progressively varies from a value substantially equal to that of said major region to a value substantially equal to that of said coupling means as said coupling means is approached, said energy coupling means being coupled directly to one of said fingers.

10. In combination, a periodic interdigital slow wave energy propagating structure having a plurality of interdigital fingers and constructed to produce along a path adjacent thereto fields of electromagnetic wave energy being transmitted, means for directing a beam of electrons along said path in energy-exchanging relation with said wave fields, external energy coupling means coupled to said structure, said structure including a major region, and means for matching the characteristic impedance of said major region to the characteristic impedance of said energy coupling means, said means for matching including a transition region disposed between said major region and said energy coupling means, said transition region having a portion wherein the pitch of the fingers included therein is constant and the height and thickness thereof is progressively varied, said transition region further having a portion wherein the thickness of .the fingers included therein is maintained constant and the pitch thereof is progressively varied, said energy coupling means being coupled directly to one of said fingers.

References Cited in the file of this patent UNITED STATES PATENTS 2,643,353 Dewey June 23, 1953 2,687,777 Warnecke et a1. Aug. 31, 1954 2,708,235 Pierce May 10, 1955 2,786,959 Warnecke et a1 Mar, 26, 1957 2,808,538 Cutler Oct. 1, 1957 2,888,598 Palluel May 26, 1959 2,895,071 Kompfner July 14, 1959 2,905,859 Osepchuk et al Sept. 22, 1959 2,942,142 Dench June 21, 1960 

